Storage batteries now in use include lead acid battery, nickel-cadmium battery and silver oxide-zinc battery. Presently, these batteries are selected for use according to the particular intended application. Recently, applications of storage batteries have expanded, e.g., they are used as a power source for electric automobiles, from the viewpoint of overcoming environmental pollution problems, and as portable power sources with respect to miniaturization and weight reduction in use of electronic equipment. Improvement in high energy density and in rapid charge performance characteristics is very desirable for storage batteries for such applications. Although development of various storage batteries, e.g., nickel-zinc battery and sodium-sulfur battery, has been directed towards high energy density, it is very difficult to put them into practical use.
Since nickel-zinc battery can have a high energy density as compared with conventional lead acid battery and nickel-cadmium battery, the battery would be expected to provide desired characteristics. However, in the battery there are various unsolved problems, such as dendrite growth, shape change, passivation, and densification of zinc negative electrodes. Sodium-sulfur battery cannot avoid difficulties in achieving a long life solid electrolyte.
Therefore, in reality only lead acid and nickel-cadmium batteries are in practical use as storage battery. Nickel-cadmium battery is highly reliable as compared with lead acid battery and also can be easily sealed, and thus the battery has recently come into wide use as portable power sources for video receivers, toys, and the like. When the battery is overcharged, oxygen evolves on the positive electrode. Although the evolving oxygen gas is consumed by combination with cadmium of the negative electrode, it is necessary to increase the partial pressure of oxygen to accelerate the rate of oxygen consumption in a cell so that sealed nickel-cadmium battery must now be manufactured using high pressure battery cases. Increasingly rapid charging can be carried out with an increase in pressure in the battery case. Generally, for a sealed type battery as charged rapidly at a rate more than 0.3 c, where the letter "c" is used to describe current rates in terms of a fraction of the capacity rating of the battery, a cylindrical battery case made of nickel-plated iron plate and withstanding high pressure is used, and the case is provided with a safety valve operating at pressure of 5 to 15 kg/cm.sup.2.
For increasing the energy density of nickel-cadmium batteries, a rectangular battery case is more advantageous than the cylindrical type one, and this is particularly the case when two or more battery cases are connected for use together. With a rectangular case, however, when the pressure becomes more than about 1 kg/cm.sup.2, even an iron battery case may be deformed, and a battery case made of synthetic resin may be broken. For prevention of such problems, such rectangular cases are provided with a safety valve operating at low pressure. However, when the battery is charged rapidly, the liquid leaks through the safety valve, soiling the peripheral part of battery and shortening the cycle life of the battery. Therefore, in practice it is actually impossible to overcharge a sealed type nickel-cadmium battery employing a rectangular battery case at 0.3 c rate and over without damage.
For prevention of overcharging, a method has been considered for stopping the charging of a battery when a voltage at which oxygen begins to evolve from the positive electrode is reached. However, the method lacks reliability, because there is only a small difference between the potential for oxygen evolution at the end period of charging and the potential of the charge process, and the difference becomes smaller when the temperature rises.
There is also a method for stopping charging involving detecting oxygen evolving from the positive electrode by so-called third electrode. However, the ability for detecting oxygen evolving from the positive electrode lacks reliability, and also it is troublesome to insert the third electrode into the battery.
When the nickel-cadmium battery is charged rapidly without overcharging, and is repeatedly used, two other significant problems are encountered.
First, the nickel hydroxide which is the active material of the positive electrode has a low charge efficiency, and it does not recover its original charged state unless at least 110 to 120% of the preceding discharge capacity is charged thereto.
Second, in the positive electrode comprising nickel hydroxide, when it is not used for a long time, a phenomenon termed an "aging effect" occurs, and oxygen evolves from the time of the initial period of charging, significantly lowering the charge efficiency and decreasing the discharge capacity, is the other problem. To overcome the "aging effect", generally the battery must be overcharged to the point of oxygen evolution, and repeat charge and discharge must be carried out.
Thus, it is in practice actually impossible to rapidly charge the rectangular nickel-cadmium battery at 0.3 c rate and over, and therefore the successful practical use of such a battery has not yet been fully realized so far. Other batteries employing nickel hydroxide positive electrodes, e.g., nickel-zinc battery, also have such unsolved problems. As mentioned above, the zinc negative electrode itself has a basic problem that its cycle life is short even when the battery is charged at a low rate under 0.1 c.